The Role of Metal Substitution in Tuning Anion Redox in Sodium Metal Layered Oxides Revealed by X‐Ray Spectroscopy and Theory

• Published in: Angewandte Chemie 2021-05, Vol.133 (19), p.10975-10982
• Main Authors: Abate, Iwnetim, Kim, Se Young, Pemmaraju, C. Das, Toney, Michael F., Yang, Wanli, Devereaux, Thomas P., Chueh, William C., Nazar, Linda F.
• Format: Article
• Language: English
• Subjects: anion redox; Anions; Chemistry; Copper; Distribution functions; Electrochemistry; Electrodes; Inelastic scattering; Jahn-Teller effect; ligand metal charge transfer; Materials substitution; nonbonding O 2p state; Oxygen; Rechargeable batteries; Sodium; sodium ion batteries; Spectroscopy; structure-redox coupling
• ISSN: 0044-8249; 1521-3757
• DOI: 10.1002/ange.202012205

We investigate high‐valent oxygen redox in the positive Na‐ion electrode P2‐Na0.67−x[Fe0.5Mn0.5]O2 (NMF) where Fe is partially substituted with Cu (P2‐Na0.67−x[Mn0.66Fe0.20Cu0.14]O2, NMFC) or Ni (P2‐Na0.67−x[Mn0.65Fe0.20Ni0.15]O2, NMFN). From combined analysis of resonant inelastic X‐ray scattering and X‐ray near‐edge structure with electrochemical voltage hysteresis and X‐ray pair distribution function profiles, we correlate structural disorder with high‐valent oxygen redox and its improvement by Ni or Cu substitution. Density of states calculations elaborate considerable anionic redox in NMF and NMFC without the widely accepted requirement of an A‐O‐A′ local configuration in the pristine materials (where A=Na and A′=Li, Mg, vacancy, etc.). We also show that the Jahn–Teller nature of Fe4+ and the stabilization mechanism of anionic redox could determine the extent of structural disorder in the materials. These findings shed light on the design principles in TM and anion redox for positive electrodes to improve the performance of Na‐ion batteries. The link between the high‐valent structure of P2‐type sodium layered transition metal oxides and the redox properties involving oxygen redox is demonstrated. By substituting Fe with Cu and Ni from P2‐Na0.67Mn0.5Fe0.5O2, the antisite‐vacancy defect formation is controlled during desodiation of the positive electrodes. Ligand to metal charge transfer and the O 2p state near the Fermi level evoke and stabilize oxygen redox from the electrodes.